1. Field of the Invention
This invention relates to new contact adhesives which are designed as multifunctional contact reactive adhesives and which may be used in a multistage curing process, for example for bonding wood, plastic moldings, metals and the like. The invention also relates to a process for the production of the new reactive contact adhesives, describing in particular a class of bifunctionally substituted urethane oligomers for the stated application. One of the functional substituents is characterized by the presence of an olefinic double bond which is accessible to radical reaction initiation. The other functional substituent is an isocyanate group which reacts off, for example, under the effect of moisture. These different reaction mechanisms enable the reactive contact adhesive to be cured in stages, particularly since the reactions involved take place at different rates, which opens up the possibility described in detail hereinafter of forming contact-sensitive adhesive layers on hard surfaces in a first stage, subsequently fitting the correspondingly coated surfaces together and firmly bonding them together in the final stage of the two-stage curing step, more particularly under the effect of atmospheric moisture.
2. Discussion of Related Art
Urethane-based coating materials curing in two stages have been described in various embodiments, cf. for example S. Peeters et al. "OVERVIEW OF DUAL CURE POSSIBILITIES IN UV COATINGS", POLYMERS PAINT COLOUR JOURNAL 179 (1989), 304-309. This literature reference describes in particular urethane-based dual-cure coating compositions which, in a first cure stage, can be cured through the presence of UV-curable acrylate groups to form a firm, but still stucturally formable or compressible material which is irreversibly cured in a following second stage. In the processing of these multifunctional materials, particular use can be made of the fact that the reactive acrylate groups can be rapidly cured, for example by UV irradiation, while other groups present in the system, which react to heat and/or moisture, cure comparatively slowly. More specifically, the cited literature reference describes systems curing in two stages by different mechanisms for use in the printing and coating industry. There is no reference to adhesives, especially contact adhesives reacting in stages which correspond to the teaching according to the invention.
EP B1 194 360 describes binders based on epoxy resins which can be crosslinked by irradiation and subsequently cured by heat treatment. More specifically, the document in question describes corresponding binders which are obtained by reaction of an epoxide compound containing more than one epoxide group per molecule with one or more carboxylic acids in a molar ratio of the epoxide groups to the carboxyl groups of 1:0.5 to 0.9 and subsequent reaction of the reaction product with one or more unsaturated isocyanatocarbamate esters which are obtained by reaction of compounds containing at least two isocyanate groups with hydroxy-functional acrylates, methacrylates or cyanoacrylates. These UV-precrosslinkable and thermally post-curing binder systems are said to be used, for example, as etching resists and/or as solder stop lacquers.
EP B1 108 630 describes radiation-curable liquid adhesive compositions for the production of clear glass laminates containing (a) a urethane acrylate component in (b) a liquid alpha,beta-ethylenically unsaturated diluent component containing acrylic acid and a monoester of acrylic acid, the composition optionally including (c) a photoinitiator. The teaching according to this document is characterized by the use of urethane acrylate components obtainable by reaction of a hydroxyl- or isocyanate-terminated oligomeric urethane intermediate product with an alpha,beta-ethylenically unsaturated compound which is reactive to the terminal groups of the intermediate product. If isocyanate groups are present as binding-promoting terminal groups in the end product, they are reacted off to prepare the urethane acrylate components and are therefore removed as reactive groups from the following curing process.
U.S. Pat. No. 4,174,307 describes a comparable reaction mechanism for binding radical-curable acrylate groups to a basic molecule via free isocyanate groups intermediately present on that basic molecule. In this case, too, starting components polyfunctionally substituted by isocyanate groups are reacted with acrylate compounds which, in addition to their radical-reactive olefinic double bond, contain a reactive hydrogen atom (as determined by the known zerewitinoff method) which leads to addition of the acrylate component onto the free isocyanate group. Active hydrogen atoms of this type are made available in particular by hydroxyl groups of acrylate monomers, although other H-active substituents, such as mercapto, imino, amino, carboxyl, carbamoyl, sulfo, sulfonamido, thiocarbamoyl groups and similar groups are suitable as potentially isocyanate-reactive groups for binding the acrylate units. According to the teaching of this document, all the isocyanate groups present in the intermediate product are supposed to be completely reacted with the reactive acrylate compounds so that polyfunctional acrylate urethane oligomers with no free isocyanate groups are formed. Various applications are proposed for the polyfunctional components of this type, including their use as adhesives.
Finally, U.S. Pat. No. 4,820,745 describes pressure-sensitive contact adhesives free from solvents and other volatile constituents, such as unsaturated monomers, based on selected polyesters of which the free hydroxyl groups are partly reacted off with acrylate or methacrylate compounds. In this case, too, free isocyanate groups inter alia may be used as reactive intermediates for binding the acrylate or methacrylate units, but are subsequently destroyed during the binding of those parts of the molecule which contain reactive acrylate groups. In addition, the document in question again relates to permanently pressure-sensitive contact adhesives and not to reactive contact adhesives corresponding to the teaching according to the present invention.